This invention refers in particular to a centrifugal separator intended for cleaning of crankcase gases from a combustion engine, especially for the driving of vehicles. Such centrifugal separators typically comprise a stationary casing and a rotor provided in the casing, which rotor comprises a spindle and a number of conical separating discs. The rotor in this kind of centrifugal separator is not normally enclosed in any casing but the oil separated from the crankcase gases is thrown outwardly against the inner wall of the stationary casing and may then flow along the wall down to a collection groove and an outlet. The rotor may be driven by various kinds of drive members. One variant is to drive the rotor by means of a blade wheel which is rotated by an oil jet from the oil system of the combustion engine. The pressure in the oil system will then determine the rotary speed of the rotor. However, such a drive of the rotor gives no indication of the rotary speed of the rotor. In order to enable any kind of control of the rotary speed, some types of sensor is required which can provide a value corresponding to the number of revolutions of the rotor. One proposed kind of sensor may include optical marking on the rotor, for instance using a bar code, and an optical reading device for reading and providing a value of the number of revolutions. Such a rotary speed sensor has, however, the disadvantage that the reading device has to include at least some part projecting in through the casing. It is then difficult to ensure high reliability regarding operation and the read number of revolutions. Furthermore, it is desirable that no such passages be made through the casing since they give rise to leakage.
EP-A-1 247 584 discloses a centrifugal separator having a stationary casing and a centrifuge rotor comprising a rotating casing and conical discs provided inside the casing. EP-A-1 247 585 discloses various different designs of sensors for sensing the rotation of the centrifuge rotor. A first variant comprises one single permanent magnet, which is fixedly attached to the centrifuge rotor and a sensor provided in connection to the stationary casing of the centrifugal separator. The sensor comprises a winding and a LED which flashes each time the permanent magnet passes the sensor. This document thus does not refer to any sensor of the number of revolutions in the sense that an exact number of revolution is disclosed, but rather an indication is given that the rotor rotates. The sensor is provided in such a way that it extends into the inner space inside the stationary casing.
U.S. Pat. No. 4,700,117 discloses a device for manipulating a high speed centrifuge. This centrifuge is a so-called laboratory centrifuge. The device aims at controlling the operation of the centrifuge and providing protection against a too high speed and to detect imbalances. The device includes magnets provided on the rotor and a stationary sensor operating with the Hall effect.
EP-A-494 421 also refers to a so-called laboratory centrifuge. Also this centrifuge has magnets on the rotor and a stationary sensor operating with the Hall effect.